This invention relates to improvements of an electric power transmission line constituted by electric cables and more particularly to an electric power transmission line constituted by electric cables wherein the sheaths of the cables of the adjacent sections are cross-connected.
In a cable power transmission line in which three single core power cables each provided with a metal sheath are laid in parallel, a cross-connection system is often adapted for the purpose of decreasing circulating current flowing through the metal sheaths.
According to the cross-connection system cables sections are connected such that two sets of ordinary junction boxes electrically interconnecting the sheaths of adjacent cable sections and two sets of insulating junction boxes insulatively interconnecting the sheaths of adjacent cable sections are used and that two sets of insulating junction boxes are included in each unit line section extending between two sets of ordinary junction boxes. The ordinary junction boxes are grounded, whereas at the insulating junction boxes, the sheaths of the cable sections belonging to different phases are cross-connected by connecting wires so as to cause the currents induced in the sheaths of the cable sections belonging to different phases to cancel with each other.
In such cross-connection system, the object thereof can not be attained unless the spacings between adjacent junction boxes in each unit line section are equal and the relative arrangement of the cables of respective phases between respective junction boxes is equal. In actual cable power transmission lines owing to the ground surface conditions or the like it is extremely difficult to satisfy these requirements with the result that considerably large circulating current flows through the cable sheaths even when the cross-connection system is adopted.
We have already developed an improved cross-connection system in which a saturable reactor manifesting remarkable saturating characteristic for large current such as fault current is connected in series with each of three cross-connecting wires at either one set of the insulating junction boxes. Although this improved system can greatly reduce the circulating current flowing through the cable sheaths even when the spacings between adjacent junction boxes are not equal or the relative arrangement of the cables of respective phases between adjacent junction boxes is not equal. However, since the normal impedance of the return circuit for the sheaths is high, a large proportion of the zero phase component of the phase current flows through the returns circuits thereby increasing the inductive interference to near-by communication lines.
As a result of our investigation made for the purpose of obviating the difficulties just described we have found that even when saturable reactors are inserted in only one or two cross-connection wires instead of inserting them in all of three cross-connection wires it is possible to greatly decrease the current circulating through the sheath of the cable of the phase in which the saturable reactor is not connected.